When landing an unmanned aerial vehicle (UAV), such as an unmanned helicopter, it is important that the aerial vehicle has access to accurate information regarding movement, position and attitude relative to the ground or landing area. In particular, when trying to land on a platform of a ship, the landing process is more difficult since the sea continuously changes the attitude of the ship and the landing platform. Also the fact that the ship is moving in a specific bearing further complicates the landing on the platform. The inclination of the platform is determined by the pitch and the roll, and there is also a movement in the vertical direction. For example, if there is a heavy sea, the amplitude of the waves can change the distance between the ship and the UAV by quite a large amount. It is important that an accurate prediction of the movements have been made before landing so that the right occasion can be chosen.
WO 2008/061307 discloses a method, which has application in the landing of a helicopter by determining the attitude of a remote surface wherein a laser transceiver is arranged to direct and detect a laser beam by the use of a mirror, which is used to rotate the emitted laser beam in order to trace out a conical surface. The base of the conical surface describes an oval shape on the landing area, and at different rotational positions a number of samples are taken. After a complete scan each 3D-point is stored into a buffer in a memory and passed to a software subroutine which calculates the plane of best fit to the stored points whereby the future positions of the remote surface can be predicted which is used in situations such when the landing platform is a moving deck of a ship.
EP 1 555 545 A1 discloses a multi-beam rangefinder system comprising a transmitter arrangement in which pulses are produced sequentially in three fixed non-coplanar directions, and the points which the pulses arrive at the ground define a plane. A planar model passing through these points then approximates the ground whereby it is possible to estimate range derived values including an altitude of the airborne platform relative to the ground, a time to land on the ground, roll and pitch as well as the angular velocity and angular acceleration.
A problem with the systems according to prior art is that they are more vulnerable and less flexible to disruptions. If one transmitter for some reason is disabled the whole system is more or less knocked-out. Furthermore if the helicopter is too high up in the air there is a risk that the tracking cone according to WO 2008/061307 is too large and misses the landing platform completely. A similar thing could happen in the system according to EP 1 555 545 A1 where any or all the pulses in the three fixed non-coplanar directions also could miss the landing platform.
Thus there is a need of a less vulnerable and more flexible arrangement in order to be able to land unmanned aerial vehicles on a surface, such as e.g. a moving landing platform of a ship.